CO 2 CAPTURE BY OXY-COMBUSTION IN CFB (Continued)

Transcription

1 68 th IEA Technical Meeting, Beijing, China, May 2014 CO 2 CAPTURE BY OXY-COMBUSTION IN CFB (Continued) Bo Leckner Chalmers University of Technology, Göteborg, Sweden and Alberto Gómez-Barea, University of Seville, Spain 1

2 CONTENT Previous presentation in Daejeon, Korea, is partially repeated (Because of the extended audience). The extention deals with the role of excess air (the previous presentation was simplified to stoichiometric conditions) 2

3 GENERAL CONDITION FOR OXY-COMBUSTION CO 2 should be removed in a pure form for deposition. This is achieved by replacing N 2 in air by CO 2, i.e. by using pure O 2 diluted by CO 2 from the flue gas instead of air to obtain reasonable combustion conditions. 3

4 GENERAL CONDITION FOR CFB BOILERS In oxy-combustion the same reasonable CFB conditions as in air-combustion should be maintained: Bed temperature 900 o C Fluidization velocity 5 m/s Excess oxygen in furnace 3-4 %O 2 Given fuel input (power) F kg combustibles/s 4

5 OXY-FUEL CFB BOILERS Air (O 2 +N 2 ) is replaced by O 2 +CO dry recirculation or by O 2 +CO 2 +H 2 O ----wet recirculation. 5

6 GAS DATA The data for the gas in the air and the oxy-fuel cases are slightly different. Gas Density ρ kg/m 3 Specific heat, C pg Mean specific heat, C pgm Molecular mass, M 900 o C kj/kg,k kj/kg,k kg/kmol Air o C 900 o C o C O N CO H 2 O

7 GAS FLOW THROUGH AN FB The flue-gas flow and the fluidization-gas flow are identical Fg = Auρ Fuel, F Fg Auρ ( T bed ) F feed rate of fuel (kg/s) g=combusion gas+nitrogen+ excess oxygen+moisture; (kg/kg) the flue-gas yield A cross section of the bed (m 2 ) u fluidization velocity (m/s) ρ gas the given composition and temperature (kg/m 3 ) To satisfy the general CFB conditions (u and T const.) the bed cross-section has to be adjusted. Bo Leckner 7 Air

8 MASS BALANCES OVER AN AIR-FIRED AND AN OXY-FIRED BOILER (related to the input of 1 kg dry combustibles) B. Leckner a, A. Gómez-Barea, Oxy-fuel combustion in circulating fluidized bed boilers, Applied Energy 125 (2014)

9 THE SPECIFIC GAS YIELD g out Combustion Nitrogen( air ) products Recirculation gas Moisture Excess oxygen cm CO hm (1 2 H2O w λ xo 2, in) = ( λ 1) + M 2 M b ( x x ) C H O, in O, r or In air-firing without gas recirculation (x o2,r =0 and x o2,in =x o2,air ) 9

10 THE FURNACE CROSS-SECTION AREA Gas yield from combustion Fg= Fluidization gas Auρ gas (T) F fuel feed rate (kg fuel/s) given, constant u fluidisation velocity (m/s) given, constant ρ gas gas density (kg/m 3 given temp Furnace cross section ratio, A oxy / A air A co2 /A af =(g co2 /g af ) volume Internal stoichiometric ratio l' =1.20 Drygas Wet gas Inlet O 2 concentration, y O2,in, mol/mol 10

11 THE GLOBAL SR λ VS INLET O2 CONCENTRATION AT A GIVEN INTERNAL SR λ. 1.2 Global stoichiometric ratio, l 1.15 Wet gas 1.1 Dry gas 1.05 Internal stoichiometric ratio, l = Inlet O 2 concentration, y O2,in, mol/mol 11

12 HEAT RELEASE/ABSORPTION Exit gas temperature is equal to bed temperature. Heat in gas leaving Fg af c p T b Heat in gas leaving Fg co 2 c p T b Internal heat transfer Internal heat transfer Fuel power FH Fuel power FH External heat transfer Heat added with gas and fuel Fg af c p T o Air-fired case <300 MW e. Combustion heat is transferred to gas and internal heat transfer surfaces. Heat added with gas and fuel Fg co 2 c p T o Oxy case with high O 2. Smaller furnace with external heat exchangers, cooling the circulating particles and so the combustion chamber. 12

13 THE HEAT BALANCE (Equation 1) Heat supplied: FH = Heat spent: =Fgc p ΔT bed on heating of air and fuel (=heating of combustion gas)+ +heat absorption by furnace heat transfer surfaces+ +and by external heat transfer surfaces AGc ps ΔT ext (if any)+ +losses (neglected) 13

14 TWO OPTIONS FOR OXY-CFB The oxy-cfb application for CO 2 capture may have two forms: 1) CO2 capture ready : similar to present air-fired CFB boilers. The cross section A is that of the air-fired boiler, and the inlet oxygen concentration has to be adjusted (flue-gas recirculation should replace nitrogen in air). 2) New development: higher oxygen concentration. The cross section is reduced. The inlet oxygen concentration is chosen, and the furnace cross-section is adjusted to maintain u and T constant. 14

15 1) THE CO 2 -CAPTURE-READY CASE a) HEAT BALANCE (Equation 1) At constant temperatures and heat transfer, the heat balance becomes and ( Fg c T ) = ( Fg c T ) out p, mix b air out p, mix b oxy g out cm hm ( , ) CO H O w λ x = ( λ 1) + O in M 2 M b ( x x ) C H O, in O, r In air-firing without gas recirculation (x o2,r =0 and x o2,in =x o2,air ) 1 x 1 x c = x x x O2, air O2, in p, mix, air p, mix, oxy O, air O, in O, r c gives the desired oxygen inlet concentration x a given λ. 15

16 OXYGEN CONCENTRATIONS IN THE READY-TO-CONVERT CASE (to satisfy the heat balance) Outlet O 2 concentration, y O2,out, mol/mol Dry gas Wet gas Global stoichiometric ratio, l Air Inlet oxygen concentrations Outlet oxygen concentrations 16

17 b) GAS BALANCE, u=const Gas produced by combustion= gas fluidizing the bed Fg = Auρ( T, gas) [ kg / s] bed Fuel, F g is given by the heat balance. The gas density is ρ= 0.29 kg/m 3 in the air case and 0.45 in the CO 2 case. The remaining quantities in the gas balance should be constant in the air and oxy-fuel case, which is not possible. Air Fg Auρ ( T bed ) Instead, optimization between T bed and u is needed (influencing suspension density and heat transfer), supported by the choice of primary/secondary air (affecting the local u). 17

18 2) THE NEW-DEVELOPMENT CASE The design is about equal to a conventional CFB Unchanged: Operation conditions, u and T bed, and so the heat transfer coefficient bed-to-surface (same bed suspension density (ρ bed (u)) and circulation rate G(u)). Changes: Oxygen concentration: between 0.21 and 1.0 (0.60 is chosen as a maximum) The corresponding flue gas recirculation (CO 2,H 2 O) The corresponding heat release per cross-section area MW fuel /m 2. Consequences: Cross-section surface A is reduced, and additional heat removal is needed. 18

19 AIR-FIRED BOILER OF SIZE 300 MW e COMPARED WITH AN 60% O 2 (Dimensions in m). 8 7 The volumes of the two furnaces V af = 9600 m 3 V co2 = 4800 m The air fired boiler has A af =240 m 2 and g af (y o =0.21, N 2 ). The oxy-fired one has A co2 =98 m 2 and g co2 (y o,co2 =0.60, CO 2 ) 19

20 HEAT BALANCE CALCULATION (Equation 1) The air-fired case (no external heat exchangers): Internal heat transfer= FH Fgaf cpaf Tbed The oxy-fired case, including external heat exchangers AGc psolids ΔT external = FH Fcpco2gco2 Tbed ( VCO / Vaf )( FH Fgaf cpaf Tbed ) 2 With a circulation flux of G=35 kg/m 2 s this gives ΔT external =90 K 20

21 CIRCULATING FLUX VS VELOCITY MEASURED ON 300 MW e CFB BOILERS WuHaibo, ZhangMan, SunYunkai, LuQinggang, NaYongjie,,Research on the heat transfer model of platen heating surface of 300 MW circulating fluidized bed boilers, Powder Technology 226 (2012)

22 CONSEQUENCES OF EXTERNAL HEATING The cooling of the external particle flow is moderate, but with limestone in the bed recarbonization may occur. CaCO3 <=> CaO+CO2 CaSO4 <=> CaO+SO2+2O2 The actual CO2 pressure range Air combustion Equilibrium diagram CaCO 3 CaO CO 2 22

23 FLUE GAS RECIRCULATION Flue gas recirculation leads to enrichment of gases A start-up sequence taken from flame combustion 23

24 CONCLUSION ON OXY-CFB DEVELOPMENT Advantages Smaller furnace size at high inlet O 2 concentration Reduction of the global excess oxygen is possible Moderation of temperature by the bed material in oxy-cfb boilers is an advantage compared to oxy-pc. Disadvantages Air separation is expensive Unknown commercial operation performance (e.g. O 2 concentration). Neutral remark Some (minor) adjustments are needed in the heat balance in the CO 2 -capture-ready case for change between air and oxy.. 24